The blooms caused by Fibrocapsa japonica have occurred regularly in New Zea1andean coastal waters, and no fish kills and economic impacts have been reported. However, Fibrocapsa japonica in Japan killed caged juvenile fish and has been regarded as a harmful micro algae. In this comparative study a New Zealand isolate was found to be morphologically similar to a Japanese isolate, although slightly larger on average than. Optimal temperatures for growth differed, with fastest growth rates occurring at $22-24^{circ}C$ for the New Zealand strain and $16-22^{circ}C$ for the Japanese strain, with a decrease in growth rate exhibited by the latter at $25^{circ}C$. Both isolates had low salinity optima of 20-25 PSU, although they grew between 15 and 30 PSU. Growth declined significantly for both strains above 30 PSU.